An aqueous coating composition comprising resinous powder uniformly dispersed in an aqueous resin solution is, because of its well balanced characteristics of good flowability due to the aqueous resin and capability of forming a thicker film due to the powdered resin, free from the generic drawback possessed by aqueous resins of sagging and is capable of excellent pinhole resistance. Hence, it far surpasses other resins in the application characteristics thereof. However, in such a powdered resin dispersion type aqueous coating composition, there is indeed excellent application characteristics at an earlier stage, but there is a serious problem of lowered pinhole resistance of the composition, one of the important application characteristics, after storing at an elevated temperature, of the composition. To solve the question of this change of stability with time at an elevated temperature, the inventors have studied the subject in depth, and found that when a carboxyl bearing resin is selected as a powder forming resin and is combined with a basic pigment and base neutralized aqueous resin, thereby masking the carboxyl groups of said powdered resin with the said basic pigment and making them inert in the system, there is obtained a coating composition which is excellent in high temperature stability with time and storage stability, without the deterioration of application characteristics thereof. On the basis of these findings, Japanese Patent Application No. 73674/82 was filed on Apr. 30, 1982. It was published as Kokai (unexamined publication) No. 189,267 on Nov. 4, 1983. According to the said invention, both storage stability and pinhole resistance with time of the resinous powder dispersion type aqueous coating composition are indeed improved, but further improvements are still needed in both respects.
Under the circumstances, the inventors, having endeavored to obtain these results, have found that if the water insoluble resin is composed of a considerably higher molecular weight polymer produced by treating a specific type of resin with amine, the swelling dissolution of such resinous powder in the system with time is effectively controlled, and high temperature stability with time of the resultant coating composition is markedly improved because of the improvement in viscoelastic change and effective control of the curing speed thereof.
Having further developed the invention of said Japanese Kokai No. 189267, the inventors have also found that if the masking amine for the carboxyl bearing resin is selected from the member having a higher boiling temperature, the curing speed is effectively and advantageously controlled, thereby giving particularly excellent application characteristics and curing properties to the composition. Of course, the combination of the abovesaid two inventions may give particularly good results in respect of storage stability, application characteristics, curing properties and the like. On the basis of these findings, the present invention has been made.